1. Field of the Invention
The present invention relates to a method for producing metal alloys, and more particularly, to a method for producing titanium alloys such as Ti-6Al-4V (“Ti64”), a titanium-aluminum-vanadium (Ti—Al—V) alloy.
2. Description of the Related Art
Superior structural properties such as corrosion resistance, light weight and high-melting point make titanium and its alloys the material of choice for many engineering applications. The alloy Ti-6Al-4V (Ti64) is the most commonly used titanium alloy and is known as the “workhorse” of the titanium alloys. As of today, 50% of all titanium metal is used as Ti64 titanium alloy that is produced from titanium sponge. Ti64 is significantly stronger than unalloyed titanium and—in contrast to commercially pure titanium—may be heat treated to increase its strength which makes Ti64 an excellent combination of high strength, light weight, high corrosion resistance and good formability.
However, the use of titanium and its alloys is limited due to the high cost associated with their production. For example, to produce a Ti—Al—V alloy according to currently known methods, titanium sponge must first be produced by using a process known as the “Kroll process”, and then aluminum and vanadium must be added by using various melting processes. In some instances, hydraulic presses are used to form alloy compacts, which are in turn welded together to form electrodes which are melted several times in a vacuum arc remelt furnace to obtain a good quality alloy. Therefore, the cost of titanium alloys is several times higher than the original cost of titanium. Titanium alloy powder production requires further processing steps and requires alloyed titanium as the starting feedstock. Therefore, the powder cost will be 15-30 times higher than the original titanium.
Despite the cost of production, titanium alloys are the only choice of many engineering applications. Accordingly, there is a need for a new titanium alloy production process that reduces the cost significantly.
Attempts have been made to produce Ti-6Al-4V alloy directly without using titanium sponge. For example, a research paper by Gao et. al. (Study on preparing Ti6Al4V alloy from V—Ti bearing beachplacers, Energy Technology: Carbon Dioxide Management and Other Technologies, Edited by L. Li, D. P. Guillen, N. R. Neelameggham, L. Zhang, J. Zhu, X. Liu, S. N. Basu, N. Hague, T. Wang, D. E. Verhulst and A. Pandey, TMS, 2016) describes the production of Ti-6Al-4V by using traditional aluminothermic reduction with accelerants. However, a Ti64 product was never realized. Additionally, the use of booster/accelerants and additional heat generating components adds an extra cost to the process, and these materials are unsafe to handle.
US 2008/0187455A1 (Armstrong et. al), U.S. Pat. No. 8,562,715B2 (Haidar) and a research paper by J. C. Withers (Production of titanium powder by an electrolytic method and compaction of the powder, Titanium Powder Metallurgy, Science, Technology and Applications, Edited by M. Quian and F. H. Froes, Elsevier, 2015) each describe different methods of direct production of Ti-6Al-4V using a mixture of chlorides of titanium, vanadium and aluminum as precursor materials. None of these references describe the direct use of titanium and vanadium oxide bearing ores for purposes of Ti-6Al-4V production.
In general, existing literature teaches away from using aluminothermic and electrochemical methods to produce Ti—Al—V alloys directly. Moreover, fundamental thermodynamic and electrochemical theory teaches away from using these methods. It is believed that no commercial process to directly produce Ti—Al—V alloys without post-process melting/alloying currently exists.